Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Even with a very high speed ratio selected, the gearbox input speed is zero when the vehicle is stationary. Since an internal combustion engine is incapable of generating torque at zero shaft speed, some type of launch device is typically employed between the engine and the gearbox input shaft. A common launch device for automatic transmissions is a fluid coupling. A fluid coupling is a hydrodynamic torque transfer device having an impeller and a turbine in a torus shape surrounding the transmission axis. When the impeller rotates faster than the turbine, the impeller causes fluid to rotate in the torus exerting torque on the turbine and a resistance torque on the impeller. A torque converter is a fluid coupling that also includes a stator that is held against rotation. The stator redirects the flow such that the torque applied to the turbine is greater than the resistance torque on the impeller. When the vehicle is stationary, the turbine is also stationary but the impeller may be coupled to the engine crankshaft. Due to the speed difference between impeller and turbine, the impeller resists rotation of the crankshaft. The resistance torque is small enough that it does not stall the engine. However, the engine must consume additional fuel, beyond that required for unloaded idle, in order to overcome the resistance torque. A multiple of the engine torque is transmitted to the gearbox input which is coupled to the turbine.